As mentioned here Fixed group population for colliding full names. · igniterealtime/Openfire@fb9c6a3 · GitHub
hardcoding an Active Directory specific attribute is not a good idea, other ldap schemas / implementations may not have those attributes, for us it bricks the integration with OpenLDAP using rfc2307bis schema. The attribute distinguishedName is not present here, which leads to a NPE. Also - ldap attributes should always be configurable since one can use whatever schema he/she likes.
A fix that should fix this issue has been applied to both the master branch (which will be the source for a future 4.1.0 release) as well as the 4.0 branch (which will be the source for the 4.0.1 release). In approximately two hours, our nightly builds should be updated with a version that includes this fix. For the impatient: try downloading an update straight from our build server.